The Government has rights in this invention pursuant to Contract No. DE-APO3-84SF15389 awarded by the U.S. Department of Energy.
Synthetic polymer membranes are useful in a number of important industrial applications, such as in large-scale filtration and separation operations. "Anisotropic" membranes, to which the present invention is primarily directed, have a continuous gradation in pore size between one side of the membrane and the other, and are particularly useful in certain microfiltration and ultrafiltration applications.
Since their discovery in the early 1960's, anisotropic membranes have been prepared using a phase inversion technique. The classical phase inversion method involves spreading a thin coating of a polymer/solvent solution on a solid support and then coagulating the solution with a nonsolvent. The system is thus ternary, involving three components: a polymer, a solvent, and a nonsolvent. U.S. Pat. No. 3,945,926, for example, which shows the preparation of membranes composed of polycarbonates and copolymers of polycarbonates, discloses the use of the classical phase inversion method. U.S. Pat. No. 4,333,972, which shows a method of preparing anisotropic cellulose-based membranes, uses a phase inversion method as well. A disadvantage of the ternary system is that controlling the desired pore configuration, i.e. sizes, distribution, etc., is difficult. During coagulation, nonsolvent penetrates into the polymer coating, while solvent diffuses into the nonsolvent. Regions depleted of solvent and penetrated by the nonsolvent begin to phase separate, since the local compositions correspond to points inside the two-phase envelope of the particular polymer/solvent/nonsolvent ternary diagram. Interaction between the polymer, solvent and nonsolvent in the phase inversion procedure thus involves a combination of complicated transport and thermodynamic phenomena.
A simplified, binary system for preparing polymer membranes is disclosed in U.S. Pat. No. 4,247,498 to Castro, the disclosure of which is hereby incorporated by reference in its entirety. That patent teaches that it is possible to eliminate the use of the nonsolvent while nevertheless maintaining a desired pore size. Coagulation is achieved by a controlled reduction in temperature rather than by use of a nonsolvent. The polymer structures described in that patent are produced by heating the selected polymer with a compatible liquid to form a homogeneous solution, and cooling the solution on an appropriate substrate. Liquid may then be extracted to form a microporous material. The final structures are characterized by a relatively homogeneous cellular structure having cells connected by pores of smaller dimension. These microporous products are substantially isotropic, however, having essentially the same cross-sectional configuration when analyzed along any spatial plane. Thus, there is a need in the art for a simplified process of preparing anisotropic membranes from a binary solution. The present invention is primarily addressed to this need, and provides such a method.
Membranes prepared by the method of the present invention may, in addition to being either anisotropic or isotropic, depending on the process parameters selected, be either "skinned" or "unskinned." "Skinned" membranes are well-known in the art (see, e.g., U.S. Pat. No. 4,035,459) and includes a relatively nonporous outer layer with the remainder of the membrane being relatively porous. Such membranes are extensively used in energyrelated applications, such as in gas separations, seawater desalination, and concentration of biotechnology products. The surface skin typically has a thickness roughly equal to that of one cell wall, although this thickness can vary. The skin may be impervious to passage of liquids or may exhibit some degree of porosity. Generally, in skinned membrane structures, the thin dense surface layer provides adequate selectivity, while the porous substructure of the bulk imparts mechanical rigidity. This type of membrane is thus ideally suited for a number of separation operations.